hcreate_r

HSEARCH(3)                 Linux Programmer's Manual                HSEARCH(3)

NAME
       hcreate, hdestroy, hsearch, hcreate_r, hdestroy_r, hsearch_r - hash ta-
       ble management

SYNOPSIS
       #include <search.h>

       int hcreate(size_t nel);

       ENTRY *hsearch(ENTRY item, ACTION action);

       void hdestroy(void);

       #define _GNU_SOURCE         /* See feature_test_macros(7) */
       #include <search.h>

       int hcreate_r(size_t nel, struct hsearch_data *htab);

       int hsearch_r(ENTRY item, ACTION action, ENTRY **retval,
                     struct hsearch_data *htab);

       void hdestroy_r(struct hsearch_data *htab);

DESCRIPTION
       The three functions hcreate(),  hsearch(),  and  hdestroy()  allow  the
       caller to create and manage a hash search table containing entries con-
       sisting of a key (a string) and associated  data.   Using  these  func-
       tions, only one hash table can be used at a time.

       The  three  functions  hcreate_r(), hsearch_r(), hdestroy_r() are reen-
       trant versions that allow a program to use more than  one  hash  search
       table at the same time.  The last argument, htab, points to a structure
       that describes the table on which the function is to operate.  The pro-
       grammer  should treat this structure as opaque (i.e., do not attempt to
       directly access or modify the fields in this structure).

       First a hash table must be created using hcreate().  The  argument  nel
       specifies  the  maximum  number of entries in the table.  (This maximum
       cannot be changed later, so choose it wisely.)  The implementation  may
       adjust  this  value  upward to improve the performance of the resulting
       hash table.

       The hcreate_r() function performs the same task as hcreate(),  but  for
       the  table  described by the structure *htab.  The structure pointed to
       by htab must be zeroed before the first call to hcreate_r().

       The function hdestroy() frees the memory occupied  by  the  hash  table
       that  was  created  by hcreate().  After calling hdestroy(), a new hash
       table can be created using hcreate().  The hdestroy_r()  function  per-
       forms the analogous task for a hash table described by *htab, which was
       previously created using hcreate_r().

       The hsearch() function searches the hash table for  an  item  with  the
       same  key as item (where "the same" is determined using strcmp(3)), and
       if successful returns a pointer to it.

       The argument item is of type ENTRY, which is defined in  <search.h>  as
       follows:

           typedef struct entry {
               char *key;
               void *data;
           } ENTRY;

       The  field  key  points to a null-terminated string which is the search
       key.  The field data points to data that is associated with that key.

       The argument action determines what hsearch() does after an  unsuccess-
       ful  search.   This  argument must either have the value ENTER, meaning
       insert a copy of item (and return a pointer to the new hash table entry
       as the function result), or the value FIND, meaning that NULL should be
       returned.  (If action is FIND, then data is ignored.)

       The hsearch_r() function is like hsearch() but operates on the hash ta-
       ble   described  by  *htab.   The  hsearch_r()  function  differs  from
       hsearch() in that a pointer to the found item is returned  in  *retval,
       rather than as the function result.

RETURN VALUE
       hcreate()  and hcreate_r() return nonzero on success.  They return 0 on
       error, with errno set to indicate the cause of the error.

       On success, hsearch() returns a pointer to an entry in the hash  table.
       hsearch()  returns  NULL  on error, that is, if action is ENTER and the
       hash table is full, or action is FIND and item cannot be found  in  the
       hash  table.   hsearch_r()  returns nonzero on success, and 0 on error.
       In the event of an error, these two functions set errno to indicate the
       cause of the error.

ERRORS
       hcreate_r() and hdestroy_r() can fail for the following reasons:

       EINVAL htab is NULL.

       hsearch() and hsearch_r() can fail for the following reasons:

       ENOMEM action  was ENTER, key was not found in the table, and there was
              no room in the table to add a new entry.

       ESRCH  action was FIND, and key was not found in the table.

       POSIX.1 specifies only the ENOMEM error.

ATTRIBUTES
       For  an  explanation  of  the  terms  used   in   this   section,   see
       attributes(7).

       +--------------------------+---------------+------------------------+
       |Interface                 | Attribute     | Value                  |
       +--------------------------+---------------+------------------------+
       |hcreate(), hsearch(),     | Thread safety | MT-Unsafe race:hsearch |
       |hdestroy()                |               |                        |
       +--------------------------+---------------+------------------------+
       |hcreate_r(), hsearch_r(), | Thread safety | MT-Safe race:htab      |
       |hdestroy_r()              |               |                        |
       +--------------------------+---------------+------------------------+
CONFORMING TO
       The  functions  hcreate(), hsearch(), and hdestroy() are from SVr4, and
       are described in POSIX.1-2001 and POSIX.1-2008.

       The functions hcreate_r(), hsearch_r(), and hdestroy_r() are GNU exten-
       sions.

NOTES
       Hash  table  implementations  are usually more efficient when the table
       contains enough free space to  minimize  collisions.   Typically,  this
       means that nel should be at least 25% larger than the maximum number of
       elements that the caller expects to store in the table.

       The hdestroy() and hdestroy_r()  functions  do  not  free  the  buffers
       pointed to by the key and data elements of the hash table entries.  (It
       can't do this because it doesn't know whether these buffers were  allo-
       cated dynamically.)  If these buffers need to be freed (perhaps because
       the program is repeatedly creating and destroying hash  tables,  rather
       than  creating  a  single table whose lifetime matches that of the pro-
       gram), then the program must maintain bookkeeping data structures  that
       allow it to free them.

BUGS
       SVr4  and  POSIX.1-2001  specify  that  action  is significant only for
       unsuccessful searches, so that an ENTER should not do  anything  for  a
       successful  search.  In libc and glibc (before version 2.3), the imple-
       mentation violates the specification, updating the data for  the  given
       key in this case.

       Individual hash table entries can be added, but not deleted.

EXAMPLE
       The  following  program inserts 24 items into a hash table, then prints
       some of them.

       #include <stdio.h>
       #include <stdlib.h>
       #include <search.h>

       static char *data[] = { "alpha", "bravo", "charlie", "delta",
            "echo", "foxtrot", "golf", "hotel", "india", "juliet",
            "kilo", "lima", "mike", "november", "oscar", "papa",
            "quebec", "romeo", "sierra", "tango", "uniform",
            "victor", "whisky", "x-ray", "yankee", "zulu"
       };

       int
       main(void)
       {
           ENTRY e, *ep;
           int i;

           hcreate(30);

           for (i = 0; i < 24; i++) {
               e.key = data[i];
               /* data is just an integer, instead of a
                  pointer to something */
               e.data = (void *) i;
               ep = hsearch(e, ENTER);
               /* there should be no failures */
               if (ep == NULL) {
                   fprintf(stderr, "entry failed\n");
                   exit(EXIT_FAILURE);
               }
           }

           for (i = 22; i < 26; i++) {
               /* print two entries from the table, and
                  show that two are not in the table */
               e.key = data[i];
               ep = hsearch(e, FIND);
               printf("%9.9s -> %9.9s:%d\n", e.key,
                      ep ? ep->key : "NULL", ep ? (int)(ep->data) : 0);
           }
           hdestroy();
           exit(EXIT_SUCCESS);
       }

SEE ALSO
       bsearch(3), lsearch(3), malloc(3), tsearch(3)

COLOPHON
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GNU                               2017-09-15                        HSEARCH(3)
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